Magnetic materials having rectangular hysteresis characteristics



Aug. 23, 1960 R. s. WEISZ 2,950,251

MAGNETIC MATERIALS HAVING RECTANGULAR HYSTEREISIS CHARACTERISTICS MOL2/26 [/1404 "/0 5 0 @5 MOL 240 [2 54mf2; 0

Filed Sept. 19, 1955 0 6' 5 M01 2710* 5mm 705 0 3- 3 INVENTOR. Foamr 5.W952 United rates Fatent Q MAGNETIC MATERIALS HAVINGJZECTANGU- LAR HYSTERESIS CHARACTERISTICS Robert Weisz, Princeton, NJ., assignor to RadioCorporation of America, a corporation of Delaware Filed Sept. 19, 1955,Ser. No. 534,954

12 Claims. (Cl. 252-625) This invention relates to improved ferrospinelcompositions and methods of making them. More particularly it relates toimproved ferrospinel compositions having hysteresis characteristicsparticularly adapted for use in saturable reactor devices.

The term ferrospinel as used herein refers to ferromagnetic oxidecompositions, also known as ferrites, which include iron oxide and haveessentially a spinel crystal structure. Ferrospinel is more definitethan the term ferrite which has been used to refer to any compoundformed when ferric oxide is combined with a more basic metallic oxide.Ferrite is a broader term than ferrospinel and may be used to refer alsoto compounds that do not have a spinel crystal structure.

Many different ferrospinel compositions have been prepared and describedpreviously. As a class these compositions exhibit advantageous magneticproperties including high magnetic permeability and low electricallosses. Certain of the previously described materials also have a nearlyrectangular hysteresis loop and a low coercive force such that they areespecially adaptable for use as saturable reactors. For example, inmagnetic memory devices it is desirable to provide a magnetic core thatmay be saturated by a magnetizing pulse of unit magnitude butsubstantially unaffected by a magnetizing pulse of /2 unit magnitude.This requirement and the reasons for it are explained in detail in anarticle by Jan A. Rajchman in the Proceedings of the I.R.E., October1953, page 1407. Only materials having hysteresis characteristics with ahigh degree of rectangularity satisfactorily meet this requirement.Previously lmown materials having substantially rectangular hysteresisloops include ferrospinels of the manganese-magnesium system andmanganese-lithium ferrospinels including one or more of magnesium, zinc,cadmium, nickel and copper ferrospinels in solid solution.

One object of the instant invention is to provide novel ferrospinelcompositions having improved magnetic characteristics particularlyadaptable for use in saturable reactors. A further object is to provideimproved methods of making these compositions to optimize their magneticcharacteristics.

These and other objects are accomplished according to the instantinvention by ferrite compositions comprising 40-47% Fe O 4055% MnO and515% ZnO or CdO on a molar basis. ZnO and CdO are equivalents in thecomposition of the invention and may be substituded one for the other inany proportion, ZnO being preferred because of cost. The optimumprocessing temperatures are specified hereinafter for several specificcompositions with n the scope of the invention. In general, optimumresults are provided when the compositions are heated at about 1075" to1275 C. for about one hour. After the compositions are fired they arecooled relatively rapidly by withdrawing them from the furnace andexposing them to the atmosphere at ordinary room temperatures.

The invention will be described in greater detail in ice connection withthe accompanying drawing of which:

Figure l is a line graph illustrating the variation in a quality factoras a function of the zinc content of several compositions of theinvention;

Figure 2 is a line graph of a different quality factor from that shownin Figure 1 but with respect to the same compositions and as a functionof the same variant; and,

Figures 3 and 4 are line graphs of the same quality factors described inFigures 1 and 2, respectively, but showing the variations of the qualityfactors in different compositions, from those shown in Figures 1 and 2.

The compositions of the instant invention may be initially prepared in amanner similar to other ferrospinel compositions. Raw metallic oxides ortheir equivalents are mixed together and pulverized as by wet milling ina ball mill for one hour or more. Generally, it is not necessary to usethe metallic oxides themselves as raw ingredients. Equivalent quantitiesof any compounds that will decompose at elevated temperatures to yieldthe desired oxides may be substituted therefor. For example, whereextreme purity is desired it is preferred to use manganese carbonateinstead of any of the commercially available forms of manganese oxide.

The milled oxide mixture is dried and calcined by heating it at about900 C. for about one hour. After calcining, the mixture is again milledto reduce its particle size and to insure intimate mixing of the oxides.

An organic binder such as parafiin or a resin and a lubricant such asstearic acid are usually added to the mixture to facilitate molding. Theparticular binder and lubricant used are not critical. About 4% byweight of a 50% water suspension of parafi'in may be used as a binderand about 1% by weight stearic acid may be added as a mold lubricant,the weight percents being based on the total weight of the finalmixture. The mixture is molded into core bodies by any convenient methodsuch as by pressing in a die, after which the molded bodies are heatedat about 400 to 500 C. to burn off the binder and the lubricant. Thebodies are then fired for about one hour at a temperature between 1075and 1275 C. The firing temperatures are relatively critical to provideoptimum electrical properties in the compositions. A change of 25 C.almost invariably seriously affects the electrical properties of thematerials, and in some cases, a change of as little as 5 C. may reducethe electrical properties to below commercially acceptable limits. Thetime for which the bodies are held at their firing temperatures, on theother hand, has been found not to be critical. This is shown, forexample, by the composition code number 5 listed in the table, which wasfired for 18 hours and still exhibits excellent electrical properties.

A representative group of different compositions within the scope of theinstant invention is listed in the table.

TABLE Composition-M01. Percent Firing Code No. Temp., D H D/H lD/H ZnO CFe O; or MnO CdO critlcal.

1 Fired and cooled in nitrogen atmosphere.

. air, or other oxidizing atmosphere.

states at different temperatures.

About 1 lb. a the hired oxides may be laced in a steel ball mill withabout a pint of water or alcohol and milled for about one hour. The'milled mixture is calcined by heating it in air or an ordinary kilnatmosphere at about 1000 C. for about'one hour. The calcined mixture,now at least-partly reacted and comprising a substantial proportion of aferrospinel composition together with some unreacted oxides is again wetground in a ball mill for at least one hour and preferably for about sixhours. About 4% by Weight of a 50% water changes of two hysteresisloops, one of the loops being emulsion of paraffin and about 1% byweight Trigazmine oleate' are added to the mixture after it is groundand 7 dried. The parafiin serves as a binder and the oleate as a moldlubricant in the subsequent molding step. The resulting mixture is arelatively thin paste which is dried so that it may be directly moldedor cut into pellets for greater convenience in molding. The mixture inbulk or pellet form is molded by pressing in a die to form a shaped corebody such as a toroid, or annulus. The core bodies are heated at'about400 to 500 C. to burn oifthe binder and the lubricant. in a hightemperature kiln where they are heated to about 1175 C. and maintainedat that temperature for about one hour. After one hour the bodies arewithdrawn They are then placed from the kiln and exposed to theatmosphere at room temperature so that they may be rapidly cooled.

The cooling step is preferably accomplished as rapidly as can possiblybe donewithoutdamaging the core bodies. Generally, the bodies must beleft on their setter plates for cooling; otherwise they may cool sorapidly that they develop cracks due tothermal strains. If the bodiesare cooled slowly their electrical properties are usually adversely'afiected, especially if they are cooled in an to be due to the unstable,ambivalent nature of manganese.

This effect appears At the firing temperaturethe manganese, it isbelieved, assumes lattice sites corresponding to an optimum, butunknown, state of oxidation. If the bodies 'are cooled slowly themanganese has an opportunity to reorient itself in the crystal structurein less advantageous positions, or to change its condition in some othermanner due to its tendency to assume different oxidation When the bodiesare quenched, on the other hand, the manganese is frozen in the positionand oxidation state it assumed at the firing temperature.

For purposes of comparing the magnetic properties of V l the materialsof the invention test toroids' of the materials are prepared asheretofore described. They are all a made of a uniform size, having anoutside diameter of about 0.5 cm., an inside diameter of 0.3 cm. and aheight of about 0.1 cm. They are wound with a primary, input winding offive turns and a secondary, output winding of one turn, each of A.W.G.No. 33 copper wire. The

hysteresis characteristics of the materials are determined generated byan alternating current of unit magnitude, and the other one beinggenerated by an alternating current of one-half unit magnitude. In mostknown material this ratio varies according to the absolute value of theunit magnitude, some materials requiring a relatively large drivingcurrent to'maximize the ratio, others requiring a relatively smalldriving current. H is. the current, expressed as ampere-turns, thatresults in the maximum ratio, .01. optimum discrimination.

Due to variations in individual test techniques the values of D asdetermined by dilferent experimenters cannot be expected to matchperfectly. They should, however, correspond inthat, for any two givenmaterials, all methods of determining D will agree as to which one isbetter than the other. The values of D stated herein are, therefore,relative only, and are not intended as absolute values.. i a

The composition of a suitable figure of merit taking both discriminationand driving current into account is relatively severe, the second figureof merit may be preferred.

The variations of these two figures of merit are shown in the drawing asfunctions of the various compositions shown in the table. Figure 1describes the first figure of merit, D/H, as a function of the zincoxide proportion in ferrospinel compositions including manganese oxideand 45 mol. percent iron oxide. It will be noted that the quality figureis at a maximum when the zinc oxide comprises about 5% of the totalcomposition. Figure 2 illustrates the variation of the second figure ofmerit, D/H with respect to the same compositions described in Figure 1.This curve shows a relatively sharp maximum at about 10% Zinc oxide.Figure 3 corresponds in general to Figure 1 except that the compositionsof Figure 3 comprise only 40 mol. percent Fe O instead of 45 mol.percent. Again it will -be noted that in these compositions, also, theD/H figure of merit reaches a maximum at about 5% zinc oxide. Figure 4describes the variation of the second figure of merit, D/H with respectto the same compositions as described in Figure 3. It,'too shows amaximum at about 5% zinc oxide concentration.

Except for the firing'temperature the processing of the compositions ofthe invention is not critical but may be accomplished according toconventional techniques. The oxides,.for example, instead of beingground initially to pulverulent form may be precipitated from an aqueoussolution in such form that they require no further grind- There havethus been described improved ferrospinel' compositions and methods ofmaking them, which compositions. are particularly adapted for use insaturable reactors.

What is claimed is:

1. Magnetic composition having a substantially rectangular. hysteresischaracteristic prepared by firing between 1075 and 1275 C, for at leastabout onehour and then rapidly cooling a formed body consistingessentially of:

M01. percent At least one material selected from the group consisting ofZnO and CdO MnO Balance 2. Magnetic composition having a substantiallyrectangular hysteresis characteristic prepared by firing between 1075and 1275 C. for at least about one hour and then rapidly cooling aformed body consisting essentially of:

Mo]. percent F203 At least one material selected from the groupconsisting of ZnO and CdO 4-15 tangular hysteresis characteristicprepared by firing between 1075 and 1275 C. for at least about one hourand then rapidly cooling a formed body consisting essentially of on amolar basis about 45% Fe O 50% MnO and 5% of at least one materialselected from the group consisting of ZnO and CdO.

5. Magnetic composition having a substantially rectangular hysteresischaracteristic prepared by firing between 1075 and 1275 C. for at leastabout one hour and then rapidly cooling a formed body consistingessentially of on a molar basis about 45% Fe 45% MnO and 10% of at leastone material selected from the group consisting of ZnO and CdO.

6. Magnetic composition having a substantially rectangular hysteresischaracteristic prepared by firing between 1075 and 1275 C. for at leastabout one hour and then rapidly cooling a formed body consistingessentially of on a molar basis about 45% Fe O 40% MnO and 15% of atleast one material selected from the group consisting of ZnO and CdO.

7. Magnetic composition having a substantially reotangular hysteresischaracteristic prepared by firing between l075 and 1275 C. for at leastabout one hour and then rapidly cooling a formed body consistingessentially of on a molar basis about 47% Fe O 48% MnO and 5% of atleast one material selected from the group consisting of ZnO and CdO.

8. Method of making a magnetic material having a substantiallyrectangular hysteresis characteristic comprising mixing togetherpulverulent metallic oxides in the molar proportions of about 40% Fe O55% MnO and 5% of an oxide selected from the group consisting of Z n0and 6 CdO, molding said mixed oxides to form a shaped body and firingsaid shaped body at about 1075 C. for at least one hour thereby to reactsaid oxides and to sinter said body, and rapidly cooling said body fromits firing temperature.

9. Method of making a magnetic material having a substantiallyrectangular hysteresis characteristic comprising mixing togetherpulverulent metallic oxides in the molar proportions of about 45 Fe O50% MnO and 5% of an oxide selected from the group consisting of ZnO andCdO, molding said mixed oxides to form a shaped body and firing saidshaped body at about 1175 C. for at least one hour thereby to react saidoxides and to sinter said body, and rapidly cooling said body from itsfiring temperature.

10. Method of making a magnetic material having a substantiallyrectangular hysteresis characteristic comprising mixing togetherpulverulent metallic oxides in the molar proportions of about 45% Fe O45% MnO and 10% of an oxide selected from the group consisting of ZnOand CdO, molding said mixed oxides to form a shaped body and firing saidshaped body at about 1175 C. for at least one hour thereby to react saidoxides and to sinter said body, and rapidly cooling said body from itsfiring temperature.

11. Method of making a magnetic material having a substantiallyrectangular hysteresis characteristic comprising mixing togetherpulverulent metallic oxides in the molar proportions of about 45 Fe O40% Mn() and 15% of an oxide selected from the group consisting of ZnOand CdO, molding said mixed oxides to form a shaped body and firing saidshaped body at about 1275 C. for at least one hour thereby to react saidoxides and to sinter said body, and rapidly cooling said body from itsfiring temperature.

12. Method of making a magnetic material having a substantiallyrectangular hysteresis characteristic comprising mixing togetherpulverulent metallic oxides in the molar proportions of about 47% Fe O48% MnO and 5% of an oxide selected from the group consisting of ZnO andCdO, molding said mixed oxides to form a shaped body and firing saidshaped body at about 1200 C. for at least one hour thereby to react saidoxides and to sinter said body, and rapidly cooling said body from itsfiring temperature.

References Cited in the file of this patent UNITED STATES PATENTS2,549,089 Hegyi Apr. 17, 1951 2,551,711 Snoek et al. May 8, 19512,579,978 Snoek et a1 Dec. 25, 1951 2,636,860 Snoek et al. Apr. 28, 19532,640,813 Berge J1me 2, 1953 FOREIGN PATENTS 683,722 Great Britain Dec.3, 1952 697,219 Great Britain Sept. 16, 1953 518,380 Belgium Mar. 1,1953 1,093,965 r e -.-,---Y---. e 5

2. MAGNETIC COMPOSITIONS HAVING A SUBSTANTIALLY RETANGULAR HYSTERESISCHARACTERISTIC PREPARED BY FIRING BETWEEN 1075* AND 1275*C. FOR AT LEASTABOUT ONE HOUR AND THEN RAPIDLY COOLING A FORMED BODY CONSISTINGESSENTIALLY OF: